easeVarying

#110

README.md

@@ -360,6 +360,10 @@ Specifies the transition [easing function](https://github.com/d3/d3-ease) for al
 
 If a *value* is not specified, returns the current easing function for the first (non-null) element in the transition. This is generally useful only if you know that the transition contains exactly one element.
 
+<a name="transition_easeVarying" href="#transition_easeVarying">#</a> <i>transition</i>.<b>easeVarying</b>(<i>factory</i>) [<>](https://github.com/d3/d3-transition/blob/master/src/transition/easeVarying.js "Source")
+
+Specifies a factory for the transition [easing function](https://github.com/d3/d3-ease). The *factory* must be a function. It is invoked for each node of the selection, being passed the current datum (*d*), the current index (*i*), and the current group (*nodes*), with *this* as the current DOM element. It must return an easing function.
+
 ### Control Flow
 
 For advanced usage, transitions provide methods for custom control flow.

src/transition/easeVarying.js

@@ -0,0 +1,14 @@
+import {set} from "./schedule.js";
+
+function easeVarying(id, value) {
+  return function() {
+    var v = value.apply(this, arguments);
+    if (typeof v !== "function") throw new Error;
+    set(this, id).ease = v;
+  };
+}
+
+export default function(value) {
+  if (typeof value !== "function") throw new Error;
+  return this.each(easeVarying(this._id, value));
+}

src/transition/index.js

@@ -4,6 +4,7 @@ import transition_attrTween from "./attrTween.js";
 import transition_delay from "./delay.js";
 import transition_duration from "./duration.js";
 import transition_ease from "./ease.js";
+import transition_easeVarying from "./easeVarying.js";
 import transition_filter from "./filter.js";
 import transition_merge from "./merge.js";
 import transition_on from "./on.js";
@@ -64,5 +65,6 @@ Transition.prototype = transition.prototype = {
   delay: transition_delay,
   duration: transition_duration,
   ease: transition_ease,
+  easeVarying: transition_easeVarying,
   end: transition_end
 };

test/transition/easeVarying-test.js

@@ -0,0 +1,43 @@
+var tape = require("tape"),
+    jsdom = require("../jsdom"),
+    d3_ease = require("d3-ease"),
+    d3_timer = require("d3-timer"),
+    d3_selection = require("d3-selection"),
+    state = require("./state");
+
+require("../../");
+
+tape("transition.easeVarying(factory) accepts an easing function factory", function(test) {
+  var document = jsdom("<h1 id='one'></h1><h1 id='two'></h1>"),
+      transition = d3_selection.select(document)
+        .selectAll("h1").data([{ exponent: 3 }, { exponent: 4 }])
+        .transition();
+  transition.easeVarying(d => d3_ease.easePolyIn.exponent(d.exponent));
+  test.equal(transition.ease()(.5), d3_ease.easePolyIn.exponent(3)(0.5));
+  test.end();
+});
+
+tape("transition.easeVarying(factory)’ factory receives datum, index, group with the node as this", function(test) {
+  var document = jsdom("<h1 id='one'></h1><h1 id='two'></h1>"),
+      transition = d3_selection.select(document)
+        .selectAll("h1").data([{ exponent: 3 }, { exponent: 4 }])
+        .transition();
+  transition.easeVarying(function(d, i, e) {
+    test.equal(e.length, 2);
+    test.equal(d.exponent, i + 3);
+    test.equal(this, e[i]);
+    return t => t;
+  });
+  test.end();
+});
+
+tape("transition.easeVarying() throws an error if the argument is not a function", function(test) {
+  var document = jsdom("<h1 id='one'></h1><h1 id='two'></h1>"),
+      transition = d3_selection.select(document)
+        .selectAll("h1").data([{ exponent: 3 }, { exponent: 4 }])
+        .transition();
+  test.throws(function() { transition.easeVarying(); });
+  test.throws(function() { transition.easeVarying("a"); });
+  test.end();
+});
+